(1) Field of the Invention
The present invention relates to an improved connection system for a tensile element, particularly applicable to the spoke of a vehicle wheel, such as a bicycle wheel.
(2) Description of the Related Art
Heretofore, the vast majority of bicycle wheels have been constructed using steel wire spokes that are connected, at their inner end, to a central hub component and, at their outer end, to a metallic rim hoop. The spokes are generally of steel construction while the hub and rim may be of aluminum or steel construction. The spokes, hub and rim are each formed as separate components that are then joined together with mechanical connections. These steel wire spokes commonly have one headed end for connection with the bicycle hub, with an opposing end that is directly threaded to accept a spoke nipple that engages the outer rim hoop. By adjusting the threaded connection between the spoke and the nipple, the overall length of the spoke may be selectively shortened or lengthened to create a balanced pretension in the spokes of the wheel.
Bicycle spokes serve as structural tensile elements where the tension of the spoke is resisted by the compression of the outer rim hoop to create a remarkably efficient wheel structure for handling the loads associated with the operation of the bicycle. The basic technology of conventional bicycle spokes has remained unchanged for more than a century.
Cyclists are continually striving to reduce the weight and increase the efficiency of their bicycle, especially the rotating components such as the bicycle wheel. However, the steel spokes of conventional bicycle wheels are quite heavy and add significant weight to the wheel assembly.
In addition to their excessive weight, steel bicycle spokes have poor vibration-damping characteristics and tend to be very efficient at transmitting road vibration to the rider. By transmitting vibration, rather than absorbing it, the conventional steel-spoke bicycle wheel lacks in rider comfort and control.
In an attempt to reduce weight, many makers of high-end wheels are forming their spokes from thinner gage steel wire. This causes the stress in the spoke to increase and makes the wheel more prone to spoke failure due to fatigue. Further, the thinner steel wire has lower tensile stiffness, which can contribute to a reduced lateral stiffness of the wheel.
The spokes of most conventional bicycle wheels are constructed of steel wire with a sharp “J” bend close to the headed end and adjacent to the point where they pass through the hole in the flange. The “J” bend region of the spoke is considerably weaker and less ductile due to the overstress of the material during the forming required to achieve this bend. As would be expected, the “J” bend region is a common breakage point for spokes of conventional design. Spoke manufacturers have attempted to compensate for this shortcoming by thickening the wire in this region, but this solution results in considerable extra expense and weight.
The tensile forces within the spoke create relatively high stresses at their connection points and these connection points and connection hardware must therefore be capable of withstanding these stresses. In the conventional spoke connection arrangement, stresses due to the spoke tension are concentrated over a relatively small region of the hub flange, namely the small portion of the hub flange that is radially outward from the spoke hole. This requires that the hub flange construction be based on expensive, higher strength materials and the use of more expensive forming processes, such as forging, rather than less costly processes, such as die casting or injection molding. Further, these stresses require that the flange be designed with robust thickness, thus adding weight to the wheel assembly.
It is often an objective to construct wheels with spokes that are flattened along their length to create a more aerodynamic cross-section profile. With a conventional hub flange, this creates a problem where the extra wide spoke cross section must pass through the round hole in the hub flange. The common assembly method, when flattened spokes are utilized, requires the slotting or notching of each individual spoke hole in the two hub flanges to allow the spoke to pass through. This additional operation adds considerable expense and weakens the hub flange as well.
With conventional wheels, the spoke is simply passed through the flange hole until the head of the spoke contacts the edge of this hole. The result is a loose clearance fit between the hub flange and the spoke, which permits the spoke to squirm and shift inside this hole. This undesirable movement results in wear at the flange and additional flex at the rim and also causes the wheel to come out of alignment (true) rather easily.
In the last 20 years, great strides have been made in the development of very lightweight materials that also have excellent tensile characteristics. Some of the most attractive of these materials include high-performance fibers, such as carbon fiber, aramid fiber, liquid crystal fiber, PBO fiber and the like. However, when attempting to utilize them as spokes in bicycle wheel construction, these fibrous materials are far more difficult to efficiently couple or terminate than their steel-wire counterparts. This is the primary reason that the vast majority of bicycle wheels are still constructed using steel spokes.
In recent years, some attempt has been made to improve on this conventional wheel design, but the changes have been minor and still retain the same materials and basic configuration. Interestingly, many of these more modern designs are simply a rehash of inventions that are more than 90 years old. This is likely due to the fact that, aside from some more esoteric examples, these modern wheels rely on similar materials and construction techniques as those employed 90 years ago.